Spinach (Spinacia oleracea L.) is a flowering plant from the Amaranthaceae family that is grown as a vegetable. The consumable parts of spinach are the leaves and petioles from the vegetative stage. Spinach is sold loose, bunched, in pre-packed bags, canned, or frozen. There are three basic types of spinach: industry, fresh and Asiatic spinach. Within these types three different leaf types can be recognised: savoy, semi-savoy and smooth types. Savoy has crinkly and curly leaves. Flat or smooth leaf spinach has broad, smooth leaves. Semi-savoy is a variety with slightly crinkled leaves. The main market for spinach is baby-leaf. Baby spinach leaves are often of the flat-leaf variety and usually the harvested leaves are not longer than about eight centimeter. These tender, sweet leaves are sold loose rather than in bunches. They are often used in salads, but can also be lightly cooked.
Downy mildew is a major threat for spinach growers, because it affects the harvested plant parts, namely the leaves. In spinach, downy mildew is caused by the oomycete Peronospora farinosa f. sp. spinaciae (formerly known as P. effusa). Infection makes the leaves unsuitable for sale and consumption, as it manifests itself phenotypically as yellow lesions on the older leaves, and on the abaxial leaf surface a greyish fungal growth can be observed. The infection can spread very rapidly, and it can occur both in glasshouse cultivation and in soil cultivation. The optimal temperature for formation and germination of P. farinosa f. sp. spinaciae spores is 9 to 12° C., and it is facilitated by a high relative humidity. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. Fungal growth is optimal between 8 and 20° C. and a relative humidity of ≥80%, and within 6 and 13 days after infection mycelium growth can be observed. Oospores of P. farinosa can survive in the soil for up to 3 years, or as mycelium in seeds or living plants.
In recent years various resistance genes have been identified that provide spinach plants with a resistance against downy mildew. However, it has been observed that previously resistant spinach cultivars can again become susceptible to the fungus. Investigations revealed that the cultivars themselves had not changed, and that the loss of downy mildew resistance must therefore be due to P. farinosa overcoming the resistance in these spinach cultivars. The downy mildew races (also called physios, strains or isolates) that were able to infect resistant spinach cultivars were collected in a differential reference set, which can be used to test spinach cultivars for resistance. A differential set also exists of spinach cultivars (hybrids) that have different resistance patterns to the currently officially denominated pathogenic Peronospora farinosa f. sp. spinaciae races.
To date 15 pathogenic races of spinach downy mildew (Pfs) have been officially identified and characterized, and many new candidates are observed in the field. The 15 officially recognised races of Peronospora farinosa f. sp. spinaciae, are designated Pfs:1 to Pfs:15 (Irish et al. Phtypathol. Vol. 98 pg. 894-900, 2008; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Benoeming van Pfs: 14, een nieuwe fysio van valse meeldauw in spinazie”, Sep. 19, 2012; Report Jim Correl (Univ. Arkansas) and Steven Koike (UC Cooperative Extension, Monterey County), “Race Pfs: 14—Another new race of the spinach downy mildew pathogen”, Sep. 18, 2012; Plantum NL press release, “Denomination of Pfs: 15, a new race of downy mildew in spinach”, Sep. 2, 2014). Races 4 to 14 were identified between 1990 and 2012, while only recently another new Peronospora isolate has been identified, termed UA4712, which subsequently has been officially named Pfs:15 by the International Working Group on Peronospora (IWGP) (Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Denomination of Pfs: 15, a new race of downy mildew in spinach”, Sep. 2, 2014. All 15 officially recognized Pfs races are publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA, and also from NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen, the Netherlands.
These newly identified Peronospora races can break the resistance of many spinach varieties that are currently used commercially worldwide, and they thus pose a serious threat to the productivity of the spinach industry. Spinach reference variety Viroflay is susceptible to all known races, while cultivars such as Lion and Lazio show resistance to multiple races. However, it is crucial to stay at the forefront of developments in this field, as Peronospora continuously develops the ability to break the resistances that are present in commercial spinach varieties. For this reason new resistance genes are very valuable assets, and they form an important research focus in spinach breeding. The goal of spinach breeders is to rapidly develop spinach varieties with a resistance to as many Peronospora races as possible, including the latest identified races, before these races become wide-spread and can threaten the industry.
Recently new Peronospora farinosa f. sp. spinaciae isolates have been identified, termed UA1014 and US1508. Along with the 15 other officially recognized Peronospora races these isolates are available from Rijk Zwaan, Burgemeester Crezéelaan 40, 2678 KX De Lier. Isolate UA1014 is also available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA. Both Peronospora farinosa f. sp. spinaciae isolates UA1014 and US1508 have been reported to the NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen as candidates for official denomination as new Peronospora farinosa f. sp. spinaciae races.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.